An organic EL device using an organic substance is a promising solid-state emitting type inexpensive and large full-color display device, and has been extensively developed. An EL device generally includes an emitting layer and a pair of opposing electrodes holding the emitting layer therebetween.
In the EL device, electrons and holes are injected into the emitting layer from a cathode and an anode respectively upon application of an electric field between the electrodes. The electrons and the holes recombine in the emitting layer to produce an excited state, and the energy is emitted as light when the excited state returns to the ground state whereby the EL device emits light.
Various configurations have been known as the configuration of the organic EL device. For example, use of an aromatic tertiary amine as a material for a hole-transporting layer has been disclosed for an organic EL device having the device configuration of “indium tin oxide (ITO)/hole-transporting layer/emitting layer/cathode” (see JP-A-63-295695). This device configuration achieves a high luminance of several hundreds cd/m2 at an applied voltage of 20 V or less.
It has been reported that an emission efficiency of about 40 lm/W or more is achieved at a luminance equal to or less than several hundreds cd/m2 by using an iridium complex (phosphorescent dopant) as a dopant for an emitting layer (see Tsutsui et al., “Japanese Journal of Physics”, Vol. 38 (1999), p. 1502-1504).
However, since most phosphorescent organic EL devices emit green light, a phosphorescent organic EL device which emits blue light in various colors has been demanded. Moreover, an increase in the efficiency of the phosphorescent organic EL device has also been demanded.
When applying the organic EL device to a flat panel display or the like, the organic EL device is required to exhibit improved emission efficiency and reduced power consumption. However, the above-mentioned device configuration has a disadvantage in that the emission efficiency significantly decreases accompanying an increase in luminance. Therefore, it is difficult to reduce the power consumption of the flat panel display.
The invention was achieved in view of the above-described situation. An object of the invention is to provide a phosphorescent organic EL device which exhibits high current efficiency and high luminous efficiency.